Patients with seasonal pollen induced asthma are to be treated by immunotherapy with either standardized pollen extract appropriate to their allergy or placebos. Changes in symptom reporting of hayfever and asthma during the season, in immediate and late responses to bronchoprovocation with allergen, in non-specific hyperreactivity to methacholine before and after antigen challenge, in immediate and late phase skin responses and in IgE and IgG antibodies will be followed.